1. Field of the Invention
The present invention relates to method and apparatus for finding the digital value requiring to drive a motor of an optical disk device and applying a driving voltage according to the digital value to the motor.
2. Description of the Related Art
FIG. 1 depicts a block diagram of an optical disk device equipped in a personal computer (PC), which comprises an optical pickup 11 for reading recorded digital signals through detecting a laser beam reflected across pits formed in the surface of an optical disk 10 and converting them into electric signals; a spindle motor 12a for rotating the inserted optical disk 10; a sled motor 12b for moving the optical pickup 11 radially on the disk 10; an R/F unit 20 for equalizing and shaping analog RF signals detected by the optical pickup 11 into binary signals; a servo unit 30 for controlling the optical pickup 11 and the rotation of the optical disk 10 based upon a focussing error signal and a tracking error signal yielded from the R/F unit 20; a motor driver 31 for driving the spindle motor 12a and the sled motor 12b separately; a voltage regulator 40 regulating the operating voltage supplied from an external source such as a PC slot to provide a constant voltage for signal processing; a digital signal processor (DSP) 50 for processing the binary signals shaped from the R/F unit 20 to restore them into original digital data; a microcomputer 60 for outputting a digital control voltage for driving the motors 12a and 12b and controlling the above elements while reproducing the optical disk 11; and a D/A converter 70 for converting the digital control voltage from the microcomputer 60 into an analog voltage.
The above conventional optical disk device equipped in a PC performs reproduction process with the power supplied from the PC slot. When an optical disk 10 is inserted in a tray (not figured) equipped in the optical disk device and clamped by a clamper (not figured), the microcomputer 60 outputs control voltages to drive the spindle motor 12a and the sled motor 12b respectively through the motor driver 31, thereby the spindle motor 12a rotates the optical disk 10 at a constant speed of 2,500 rpm owing to the driving voltage supplied from the motor driver 31. At the same time, the sled motor 12b, which is a stepping motor, moves the optical pickup 11 to an initial reproduction position of the optical disk 10. This operations done, the reproduction process starts.
During the reproduction, the optical pickup 11 should be moved radially on a request for search or movement. In this case, the microcomputer 60 controls the sled motor 12b. The detailed control process of the microcomputer 60 is as follows.
If the microcomputer 60 intends to apply a certain driving voltage, for example 1.5V, to the sled motor 12b, the microcomputer 60 applies the value of 01001100.sub.(2) which is a digital value of 76(=1.5.div.5.times.255) to the D/A converter 70 because the microcomputer 60 divides the external operating voltage of 5V into 255 steps (in case of 8 bit D/A converter), and the D/A converter 70 converts the digital value of 01001100.sub.(2) into an analog voltage of 1.5 (=76.div.255.times.5). Accordingly, the motor driver 31 generates micro step pulses corresponding to 1.5V, thereby the sled motor 12b is rotated so that the pickup 11 moves to target location.
However, the operating voltage supplied from an external source may be changed from 5.5V to 4.5V since the external power supplied from a PC slot has variance range of .+-.10%, and the microcomputer 60 cannot be aware of the changes of the supplied power. Accordingly, the microcomputer 60 still outputs the digital value for 76 to the D/A converter 70 if it is intends to apply 1.5V to the sled motor 12b even in the condition that the operating voltage has been changed as much as maximum 10%.
However, the analog voltage converted for the digital value of 76 by the D/A converter 70, which converts the input digital value into an analog voltage in the basis of the supplied operating voltage, becomes not 1.5V but 1.3V (=76.div.255.times.4.5) if the supplied operating voltage has been reduced to 4.5V. Accordingly, the sled motor 12b can not move the pickup 11 as much as desirable, which causes time delay in finding a target track since an additional movement is required.